1. Field of the Invention
The present invention relates generally to a device for delivering a therapeutic preparation into the body of a patient by injection into or through the patient's skin. More particularly, the invention relates to an automatic injection device that incorporates a self-emptying reservoir.
2. Related Art
The conventional method of administration of a drug to a subject is by injection using a hypodermic syringe. A number of difficulties associated with these syringes have led to attempts to derive more advantageous drug delivery devices. Syringes are not generally advocated for use in self administration by patients because of the dangers of embolisms arising from the introduction of air bubbles into the bloodstream, incorrect dosing, and the accidental infection of third parties after use of the syringe. In any event, syringes cannot be used by children or by many elderly patients, and the use of syringes is very traumatic for the large number of people who are needlephobic to a greater or lesser extent.
Various types of automatic injection devices have been developed to allow drug solutions and other liquid therapeutic preparations to be administered by untrained personnel. In trying to provide improved injection devices, a number of inventors have focussed on the provision of a pre-filled syringe or a pre-filled ampoule for use in a syringe, as these devices can be useful in addressing the problems of incorrect dosage or incorrect filling of syringes. Furthermore, some syringes have been provided with expelling means which automatically deliver the drug from the syringe body or ampoule, rather than relying on a conventional syringe mechanism which can be difficult to manipulate in a smooth uniform fashion with one hand.
Generally, these devices include some type of automatic needle-driving mechanism (usually of the spring-loaded type) that can be triggered by the user. Examples of such devices may be found in U.S. Pat. Nos. 4,188,950, 4,196,732, 4,258,713, 4,227,528 and 4,378,015, all to Stephen C. Wardlaw. Still further examples can be found in U.S. Pat. No. 4,214,584 to Smirnov et al., U.S. Pat. Nos. 4,894,054 and 5,527,287, both to Miskinyar, and U.S. Pat. No. 5,616,132, to Newman. In order to start the flow of the liquid therapeutic preparation when the needle is injected, the devices disclosed in the aforementioned patents generally employ movable ampoules, pistons or other complex arrangements which are somewhat difficult to manufacture.
Another class of devices includes those which are capable of gradually infusing a liquid therapeutic preparation into the skin of a patient. In some cases, these devices are small enough (both in height and in overall size) to allow them to be “worn” by an ambulatory patient while the liquid therapeutic preparation is being infused into the patient. Examples of devices which fall in to this class include those disclosed in U.S. Pat. Nos. 4,340,048 and 4,753,651, both to Eckenhoff, U.S. Pat. No. 4,734,092, to Miller, U.S. Pat. No. 4,781,688, to Thoma et al., U.S. Pat. No. 4,886,499, to Cirelli et al., U.S. Pat. No. 5,656,032, to Kriesel et al., and PCT Publication Nos. WO 95/13838 and WO 97/21457, both to Elan Medical Technologies, Ltd.
Unfortunately, most of the automatic infusion devices disclosed in the prior art are fairly complex in design and, as a result, cannot be made as small and inexpensive as might be desired. Generally, the complexity of these devices results from three factors. One factor is the need for a pump or other type of discharge mechanism to force the liquid therapeutic preparation to flow out of the reservoir and into the injection or infusion needle. Another factor is the need for some type of valve or flow control mechanism to cause the liquid therapeutic preparation to begin to flow at the proper time. A third factor, which applies to those devices that are designed to inject the infusion needle into the patient automatically, is the need for a suitable injection mechanism that can be triggered by the user. The structures required to perform these functions add size and complexity to the infusion device, making it larger than desired and relatively expensive to manufacture.
Accordingly, a need exists for a drug delivery device that is capable of delivering a pre-set dosage of drug to a subject; is suitable for use in self-administration by patients (including young patients and elderly patients); does not require the patient to consciously insert a needle into the skin; includes a safety mechanism to prevent re-use of the needle; and has a simple construction. Further objects and advantages of the invention will become apparent from the description given below.